This invention relates to a modified Karl-Fischer reagent for the determination of water, the reagent containing a pyridine substitute, sulfur dioxide and iodine, and to a process for the determination of water by means of this reagent.
A number of proposals for the replacement of pyridine in the Karl-Fischer reagent by other substances are known from the literature. In Anal. chim. Acta 94, 395 (1977), sodium acetate is used as a substitute for pyridine. However, this substitution involves certain disadvantages. For example, ethyl acetate forms by reaction with the alcohol used as the solvent, water being eliminated which naturally interferes in a method for the determination of water. The solutions are therefore unstable, and their blank value increases continuously.
In British Pat. No. 728,947, alcoholates, phenolates and metal salts of weak organic acids are also mentioned in addition to acetates as a substitute for pyridine. A check of the substances mentioned in the patent specification showed that these are unsuitable as a pyridine substitute, in some case due to inadequate solubility and in other cases due to insufficient stability of the finished solutions.
To overcome these disadvantages, attempts have been made very recently to replace pyridine by aliphatic amines in a defined molar ratio relative to sulfur dioxide, or by heterocyclic compounds (European Pat. No. 35,066; U.S. Pat. Nos. 4,429,048 and 4,378,972). However, even this pyridine substitute still has disadvantages, because the stability of the end point fluctuates with the quantity of the water to be titrated.